Add landing page for electrum ABC wallet

Summary:
Adding the landing page for the Electrum wallet

• updated link on bcha page to go to the electrum page
• download link goes to github releases
• Depends on D8504

Test Plan: Review and adjust as needed

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8505